In the area of microwave engineering, a variety of coupling devices are known for combining two or more microwave signals in a common waveguide. One particularly useful device for such signals is the orthogonal mode transducer. Essentially, orthogonal mode transducers provide for either combining or separating signals which are orthogonal to one another. Typically, this is done using a pair of rectangular waveguide arms coupled to a common waveguide arm in such a fashion that the cross-sections of the rectangular arms are perpendicular to one another.
Orthogonal mode transducers are used in a variety of communication arrangements. One common use for such orthogonal mode transducers is to apply signals of the same frequency which are polarized orthogonally with respect to one another to the rectangular arms for combination in the central arm which is capable of supporting both orthogonal polarizations. Thus, the central arm will carry a combined signal having components which are orthogonally polarized to each other. Such a device is useful for signal transmission. On the other hand, the orthogonal mode transducer can be used to receive at the common arm a signal having a pair of orthogonally polarized components. In this case, the signal would then be separated into its orthogonal components by the rectangular arms, each of which is dimensioned to support only one of the orthogonal components of the received combined signal.
Another common use for orthogonal mode transducers is in transmit-receive systems using a transmitted signal which is polarized orthogonally to the received signal, and which has a different frequency than the received signal. This latter use is especially common in satellite communication systems wherein signals are transmitted to the satellite on the up-link at one frequency and received on the down-link at a different frequency which is polarized orthogonally relative to the transmitted wave.
In the past, most orthogonal mode transducers have been constructed in a general T-configuration. This is typically done in one of two ways. The most common approach is to utilize a linear arrangement between one of the rectangular waveguides and the common waveguide with the orthogonal rectangular waveguide feeding into the common waveguide at a right angle. Thus, the common waveguide and the first rectangular waveguide form the crossbar of the T-configuration while the second rectangular waveguide forms the base of the T.
Another T-configuration is an arrangement wherein the rectangular arms form the top bar of the T while the common arm forms the base of the T. Salzberg U.S. Pat. No. 3,932,822 is an example of such an arrangement.
Although such systems are in common use, they suffer from the basic practical problem of difficulty of construction. Because of the requirements of matching arms properly for the desired wave propagation, it is difficult to properly construct a basic T-configuration to result in a simple and yet structurally strong structure.
Another problem with the types of orthogonal mode transducers discussed above is the amount of space which they occupy due to their configuration. For example, if a linear arrangement is used, a taper is required between the rectangular arm and the common arm which is in line with the rectangular arm. The other rectangular arm is connected to this tapered portion. Due to this arrangement, the length of the device is disadvantageously long.
In systems of the type shown in Salzberg, on the other hand, the perpendicular T-arrangement requires the use of 90.degree. bends at the ends of the rectangular arms in order to couple these rectangular arms to other transmission lines in the system. This occupies a great deal of width. Thus, it can be seen that both prior types of systems occupy a large amount of space and, thus, are not well suited for situations where space is at a premium.
U.S. Pat. No. 3,089,102 to Rowland illustrates one attempt to depart from the conventional T-configuration to obtain a strong rigid structure which has good separation characteristics between a transmitted wave and a received wave. Essentially, this patent shows a modification of the standard T orthogonal mode transducer of the type wherein the common waveguide (in this case a square waveguide) and one of the rectangular waveguides form the top bar of the T. However, rather than having the other rectangular waveguide form the base of the T, as is conventional, the Rowland patent has the second rectangular waveguide branching off from the square waveguide and the first rectangular waveguide at an angle other than 90.degree.. Thus, the result is a type of asymmetric Y-configuration with the square common waveguide forming its base and the two rectangular waveguides forming an asymmetrical top portion.
Although the above-described Rowland system does provide good structural strength, it is still rather difficult to manufacture it due to its asymmetric configuration. For example, a special transition flare section for converting one of the rectangular waveguides to a square waveguide is necessary while permitting coupling of the second rectangular waveguide at an angle. This creates manufacturing difficulties and also adds to the length of the device. Also, the asymmetrical arrangement of the rectangular arms creates bandwidth limitations which give poor overall response. This is particularly true if the orthogonal mode transducer is coupled to a waveguide having high order mode capabilities.